There are a variety of diamond types, such as, transparent large-sized single crystal or colored crystal diamonds which have various lattice defects, or impurities, (ie. Ni or Si) contained in the crystals. These diamonds are classified into type I diamonds with light absorption at a wavelength of 3000 angstroms or less in the ultraviolet region and type II diamonds with transmission of light at 2250 angstroms or more. A type II diamond is purer than a type I diamond.
Type I diamonds are further classified into type Ia diamonds which contain 0.1% or less of nitrogen (this type includes most of the natural diamonds) and type Ib diamonds in which nitrogen of 500 ppm or more is replaced with carbon. Type II diamonds can be classified into type IIa high resistance diamonds (free of nitrogen) and type IIb p-type semiconductor diamonds.
Diamonds have various desirable properties such as high hardiness, low thermal expansion coefficients, chemical stability (ie. inactive), high thermal conductivity, high electrical insulation, high infrared transmission, high reflectivity, high sound velocity, wide band gap semiconductor characteristics, high-speed carrier properties, color center, negative electron affinity, physiological adaptability, and superior acoustic characteristics and so on.
Based on the above referenced characteristics, diamonds are utilized in many industrial applications which include: cutting tools; wear-resistant coating materials; circuit substrates; high frequency devices; heat sinks; various optical parts; electronic device parts such as semiconductors and radiation sensors; surface acoustic wave filters; speakers; and physiological functional parts such as artificial joints.
It has been well known for many years that diamonds can be successfully synthesized under a high pressure. Therefore, diamonds have been widely utilized as industrial materials for many years. More recently, diamond thin films which have many of the above stated advantageous characteristics have attracted more attention since a technique was developed for producing diamond thin films in a gas phase.
A well known method for forming a thin film on a substrate is chemical-vapor deposition (CVD). This method includes several techniques. One method includes: setting an open quartz tube at a position close to a tungsten filament which is heated at a high temperature of about 2000.degree. C.; and introducing through the quartz tube a diluted mixed gas of hydrocarbon gas, such as methane and hydrogen, which thereby causes the diamond in the mixed gas to be deposited on a substrate heated to within a range of 500.degree. C. to 1100.degree. C. Other methods include a microwave plasma CVD technique, a radio frequency (RF) plasma CVD technique, or a direct current (DC) arc plasma jet technique which all utilize plasma discharge instead of the above described hot filament technique. Another method is to decompose and deposit diamonds from a hydrocarbon-containing gas (ie. oxygen-acetylene) at high speed in the air known as the combustion-flame method.
In the above methods, a diamond is formed by the principle that hydrogen is dissociated to atomic species, whereby graphite produced simultaneously is preferentially eliminated to grow a film of diamond structure. The mechanism of diamond formation is very complicated, and is not completely well understood at present.
These methods provide diamonds having various different qualities, manufacturing operabilities and capabilities. In addition, the actual qualities of these diamonds are very sensitive to many factors, such as, gas or electron temperature, reaction gas composition, gas pressure, reaction gas flow rate, substrate material, substrate temperature, deposition machinery, and so on, and the methods do not obtain a satisfactory level of stable and reproducible production with high efficiency.
In particular, the microwave plasma CVD technique and the RF plasma CVD technique which utilize plasma discharge each have the problem of requiring complicated and expensive devices having complicated load monitoring systems for stabilizing the plasmas. In addition, there is the disadvantage that diamond growth is very slow even when expensive devices are utilized, and high density defects or strains exist in the obtained diamond film structure.
Stresses due to tensile or compressive strain are present in diamond thin films which are formed on substrates utilizing any of the above described CVD methods. Occasionally, the diamond thin films crack or slip off the substrates.
The occurrence of such strains is caused by defects in the diamond thin film, impurities on the grain boundary and grain boundary sections, as well as by differences in the thermal expansion characteristics between the substrate and the diamond.
In addition, even if an attempt is made to obtain a diamond thin film with specified properties, a pseudo diamond with poor characteristics is formed, a diamond-like carbon thin film is formed, or quality degradation such as the generation of graphite frequently occurs.
As described above, even if at present the CVD technique is one of the effective means for forming a diamond film, a satisfactory result is not obtained. Thus, at present, a diamond thin film cannot be produced stably and reproducibly.
Attempts have been made to form diamond films on both sides of a substrate utilizing CVD techniques in order to overcome the problems related to the formation of stresses. See Japanese Patent Laid-open No. 5-306194.
However, with respect to forming films on both sides of a substrate, there is no guarantee that the same diamond film is formed correctly and uniformly on both sides of the substrate. Although the above reference describes a hope for uniformly forming a film so that unbalanced internal strain is eliminated, attempts have not proven satisfactory. Therefore, strains still exist in the crystal.
Moreover, it is difficult to ideally obtain such a diamond film unless the substrate heating temperature, gas flow direction for forming the film, and decomposition and deposition are precisely controlled to ensure that uniform diamonds are formed on both sides of the substrate. This creates an additional problem that the stated device and control method must become extremely complicated and expensive.
Another proposal has been disclosed in which diamond is annealed in an alumina furnace in an atmosphere of hydrogen or a mixed gas of hydrogen and inert gas for the purpose of reducing the intervening materials existing in synthetic diamond powders and reducing the residual strain in a diamond lattice to improve toughness. See Japanese Patent Laid-open No. 7-165494. Further, there has been an attempt to perform electron beam irradiation and heating to 1600.degree. C. to 2200.degree. C. in order to reduce the color of diamond powders. See Japanese Patent Publication No. 62-43960. There has also been an attempt to dope diamond with a high level of nitrogen, boron, arsenic, phosphorus and so on to damage the surface of diamond crystals and then to perform annealing using a coil to recover lattice damage. See Japanese Patent Laid-open No. 6-166594.
Although all of these above stated methods provide a certain effect in reducing strain, their efficiency is small, and it is not easy to restore crystals which are damaged/roughed as suggested in the last disclosed reference. In any case, all of the above methods do not provide an effective means for decreasing the residual strain of the diamond lattice.
Meanwhile, attention has also been recently given to CBN thin films, BCN thin films, and CN thin films which have the same crystal structure and similar hardness as diamonds. These thin films consist of two- or three-components and the films readily deviate from a stoichiometric composition (in CBN, there occurs a phenomenon that the composition ratio B/N fluctuates locally), and defects or strains are more likely to occur.
In particular, it is expected that the CBN film can be formed as a high-temperature semiconductor film. There is the possibility that an n-type semiconductor can be produced and that an ultra high speed carrier transistor in combination with diamond having p-type semiconductor characteristics may be producible. It is also expected that a CN film can form a harder film than diamond. However, there is still the problem that strains and defects are likely to occur during film deposition, thus making it impossible to ensure practical use.